Human T cell leukemia viruses (HTLV) are widespread and associated with specific disorders of myelopathy and lymphoproliferation, including an aggressive form known as adult T-cell leukemia/lymphoma (ATLL). HTLV can infect a broad array of vertebrate cell types, suggesting high levels of conservation of the receptor, or that several different molecules allow virus entry. Identification of the receptor(s) could provide valuable insights into the nature of virus-receptor interactions that could be targets for antivirals, improved understanding of pathogenic determinants of these viruses, and possibly tools for developing small animal models for infection. We propose to use genetic approaches to identify the HTLV receptor and genetic and biochemical approaches to characterize its activity. AIM 1. Identification of the HTLV receptor - Genetic Approach. We will use a VSV-G pseudotyped murine leukemia virus retrovirus expression library to transduce HeLa cDNAs into cell lines defective for HTLV entry. These cells will then be infected with HTLV envelope pseudotyped HIV vector expressing drug resistance markers, and selection carried out sequentially with G418, puromycin, and mycophenolic acid. Selected cells will be screened with an HTLV envelope pseudotyped HIV vector expressing luciferase, and the resultant cDNA identified by PCR and sequence analysis. AIM 2. Characterization of the HTLV receptor - Biochemical & Genetic Approaches. We will determine if the cDNA conferring entry on defective cell lines results in expression of a cell surface molecule that binds the HTLV-1 envelope glycoprotein. We will determine if siRNA to this cDNA depresses HTLV binding and infection of susceptible human cell lines. Lastly, we will determine if the identified cDNA enhances HTLV-1 envelope binding to murine cell lines, and whether this allows HTLV replication and/or immortalization of murine T cells.